Several non-intrusive inspection systems, including X-ray scanning systems and neutron interrogation systems, have been developed and deployed for the detection of conventional explosives or narcotics. However, these systems are not adequate for the detection of the fissile material of a nuclear weapon hidden in a typical cargo container. X-ray techniques cannot easily differentiate the fissile material from innocuous heavy metals such as lead, tungsten and bismuth.
The most common neutron-based technique employed to detect fissile material is differential die-away (DDA). In this method, the item to be inspected is placed in a chamber or enclosure containing a pulsed source of energetic, or fast, neutrons. The fast neutrons slow down to thermal energies, and then die away over a period of microseconds to milliseconds, depending on the thermal neutron capture properties of the environment. If fissile material is present in the item, then fission events induced by thermal neutrons will perturb the die-away characteristics of the thermal neutron fluence rate due to the addition of fission neutrons.
Consequently, by monitoring the thermal neutron fluence rate die-away time with a thermal neutron detector between fast neutron pulses, the presence of fissile material in an item can be detected. The DDA technique suffers from the fact that the thermal neutron detectors typically employed for DDA cannot process very high event rates, and a significant waiting period after the pulse is needed before counting can begin in order to allow the detectors to recover from saturation effects due to the pulse. This time delay results in a significant reduction in detection sensitivity. Furthermore, DDA can be circumvented by placing a thermal neutron absorber, such as boron, lithium or cadmium, around the fissile material.
Simple gamma spectroscopy can also be employed to detect and identify fissile or fissionable material. This method relies on the detection of the decay radiation emitted from fissile or fissionable radionuclides by a high resolution gamma ray detector. However, the most prominent radiation emitted from fissile nuclides is typically low energy gamma rays, especially in the cases of uranium 235 and plutonium 239, and can therefore be absorbed with a modest amount of gamma shielding placed around the fissile device. Thus, a system relying solely on decay gamma detection can easily be circumvented.
Previous work in this area has been limited by a lack of sensitivity of detector electronics. For work employing pulsed neutron generators and gamma-ray detection, the short pulse durations (1–5 microseconds) were characterized by extremely high instantaneous neutron emission rates, and the resulting gamma ray flux was so large that the gamma-ray detector electronics were paralyzed. In addition, all systems—whether using electronic or isotopic neutron sources—were hampered by relatively poor signal-to-background noise ratios for peaks from trace constituents in a sample.
To enhance homeland security protection, new and/or improved technologies are needed to prevent and deter the smuggling of materials that can be employed for catastrophic terrorist attacks. These materials include constituents of nuclear, conventional (i.e., explosive), chemical and radioactive weapons. Detection of illicit attempts to transport these threat materials past points of entry, such as airports, ports and borders is a key component of the fight to protect the security of U.S. and allied countries. The current non-intrusive inspection methods for the detection of fissile material are either inadequate or can readily be circumvented.